The present invention is directed to a system and method for combusting hydrocarbon fuels in an efficient manner which minimizes pollutant emissions, particularly NOx emissions.
Exhaust gases produced in the combustion of hydrocarbon fuels by engines contribute to atmospheric pollution. Exhaust gases typically contain nitric oxide (NO), nitrogen dioxide (NO2), carbon monoxide, and unburned hydrocarbons. Nitrogen oxides are a cause of smog, acid rain, and depletion of stratospheric ozone. With high combustion temperatures in an engine, oxygen and nitrogen combine to form the pollutants NO and NO2 (collectively known as “NOx”). Typical fuels reacting with air exceed the threshold temperature which results in NOx formation.
A reduction in the formation of NOx is desirable. One method to control NOx is to employ a catalyst that allows low-temperature reaction of fuel and air. Most, if not all, of the fuel can be reacted at a moderate temperature, thus inhibiting formation of NOx. The use of a catalyst results in a pre-reaction of a portion of a fuel to stabilize the main combustion process. The catalytic process is referred to as catalytic combustion.
When reacting fuel with a catalyst, heat is generated. This heat must be controlled to result in a lower combustion temperature. Typically only a portion of the total fuel to be burned is reacted in the catalyst chamber. One solution to the problem of heat production is to provide a stream of cooling air about a stream of fluid that is in contact with the catalyst or a substrate to which the catalyst is attached or resides. Such a process uses heat exchange in which certain channels contain the catalyzed fluid, while other channels contain air for cooling and absorbing heat from the catalytic reaction. These two fluid streams can then be mixed upon exiting the heat exchanger and combusted with a reduction of NOx.
In catalytic combustors (or catalytic reactors), hydrocarbon fuel is mixed with a first air stream to form a fuel and air mixture having an equivalence ratio greater than unity, that is with fuel in excess, and partially oxidized by contacting the fuel/air fluid mixture with an oxidation catalyst stage, thereby generating the heat of reaction in a partial oxidation product stream comprising hydrogen, water, and carbon oxides. The reaction is intended to be pure catalytic, thus minimizing the formation of oxides of nitrogen (NOx).
A portion of the heat of reaction is conducted through the wall of a substrate on which the catalyst resides and is removed via the back side convection and conduction heat transfer to the second air stream and/or compatible cooling fluid. The partial oxidation product stream is mixed with a second air stream, which is raised in temperature from its initial state via the heat of reaction, and subsequently combusted in a down stream combustor. The down stream combustor can include additional fuel or air mixtures that contribute to combustion in single or multiple zones.
The fuel/air mixture flows into a catalytic oxidation stage and contacts an oxidation catalyst which partially oxidizes the mixture to generate heat and a partial oxidation product stream comprising hydrogen, carbon oxides (primarily CO), and unreacted hydrocarbon fuel. Catalytic oxidation in this context is intended to drive a rapid oxidation or oxidative pyrolysis reaction carried out at a temperature below that required to support thermal combustion or combustion without a flame at a temperature below which thermal NOx will not form in appreciable amounts. Partial oxidation means that there is insufficient oxygen available to completely convert fuel to carbon dioxide and water, and thus fully liberate the chemical energy stored in the fuel. Since complete oxidation improves the energy potential of the fuel, any improvement in the catalytic reactor is beneficial.
A practical problem in the design of an integrated catalytic reactor-heat exchanger is balancing the rate of reaction and chemical heat release with the rate of heat exchanged to the adjacent cooling flow. If not well-balanced, sections of the reactor can over heat leading to premature failures or loss of activity. Such a situation often occurs at the leading edge of a reactor system. Reaction rates could be changed by altering the catalyst loading on the catalytic surface. Such a strategy is generally not acceptable, however, since system performance will be adversely affected as the catalyst degrades. It is better to design the hardware such that the rate limiting processes are based on geometry and fluid flow conditions, and hence controlled by transport processes rather than catalytic activity. This invention provides such a strategy.